Technical Field
The present invention relates to a shovel provided with an attachment and to a control method thereof.
Description of Related Art
A shovel having a variable contraction that increases and decreases a flow rate of a hydraulic oil flowing out of a rod side hydraulic chamber of an arm cylinder when closing an arm is known. This shovel monitors a pressure in a bottom side hydraulic chamber of the arm cylinder to control the variable contraction. This is because it can determine that a bucket is not in contact with a ground surface and an excavation attachment is moving in the air when the pressure in the bottom side hydraulic chamber is lower than a predetermined value, and can determine that a flow rate of the hydraulic oil flowing through the variable contraction is to be decreased lest the arm should descend under its own weight. Also, this is because it can determine that a bucket is in contact with the ground surface when the pressure in the bottom side hydraulic chamber is greater than or equal to the predetermined value, and can determine that a flow rate of the hydraulic oil flowing through the variable contraction is to be increased lest an unnecessary pressure loss should is generated at the variable contraction.
However, the above shovel can determine whether to decrease or increase a flow rate of the hydraulic oil flowing through the variable contraction only after a contact of the bucket and the ground surface has been detected based on the pressure in the bottom side hydraulic chamber of the arm cylinder. As a result, it cannot increase the flow rate at the start of excavation. Thus, it generates an unnecessary pressure loss at the variable contraction and decrease working efficiency of the shovel. This is due to the fact that it cannot preliminarily determine when the bucket gets in contact with the ground surface since it has not recognized a current shape of the ground surface to be excavated.